Buoyant filter media has been used for some time for purposes of removing solids from waste liquids and in particular wastewaters obtained from industrial and residential environments. Published U.K. Patent Application No. 2,080,696 discloses a filtration system using buoyant filter media. The filter media is contained in a vessel which permits the buoyant media to rise upwardly of the vessel until steady state for the height of the buoyant material in the vessel is achieved for a particular flow rate. An effluent weir is provided in the vessel at a level beneath the anticipated height of the buoyant material, to draw off the clarified liquid. The weir includes a screen to prevent the buoyant material from passing over the weir into the effluent outlet channel. The screen covering the channel can become clogged with the buoyant material, considerably reducing the effluent flow rate or causing an overflow in the upstream portion of the vessel, hence necessitating frequent checking on the system. Due to the screen being of relatively minimal cross-sectional surface area, compared to the size of the vessel, back washing is frequently required or spraying of the screens with pressurized water to cleanse them is also required. To provide cleansing of the majority of the buoyant material, air injection is required to break up the bed of buoyant material to facilitate cleansing of the bed of buoyant material.
Systems which involve the use of screens to maintain a desired level of buoyant material in the system are disclosed in U.S. Pat. Nos. 4,246,118; 4,446,027 and published European Patent Application No. 107,353. These systems trap the buoyant material in the treatment vessel, thereby necessitating frequent back wash and optionally the use of airstreams to loosen the buoyant material. Often the nozzles for injecting air in water streams during the back washing and cleansing operation become clogged or otherwise disabled due to deposits on the nozzles, which considerably reduce their effectiveness in breaking up and cleansing of the buoyant materials. In European Application No. 107,353, the screen for retaining the buoyant material in the vessel functions to hold above the screen a granular material which effectively retains the buoyant material beneath the screen, since the screen has an aperture sizing larger than the nominal size of the particles of buoyant material. By employing the larger granular material as the device for retaining the desired level of buoyant material in the vessel such as the use of gravel, the operative filtration time is extended. However, back washing or air-water flushing of the gravel is required once the gravel becomes clogged with collected solids.
According to this invention a filtration system involving the use of buoyant media is provided which requires less frequent maintenance of the filter media.